[Bagga]

Just one point though, with regard to the examples of negative need in the past:

we didn't need to do it = we needn't have done it

That example has meanings that may be slightly different.


we didn't need to do it can mean that a task was set, at a time in the past, but it wasn't necessary for us to do it. (That is to say, the task was scheduled to be done AFTER the point in time that the task was set.)

we needn't have done it means that the task had been completed, but it wasn't necessary. (That is, we wasted all that time and effort when we could have been down the pub discussing the finer points of grammar. Dang!)

Just my two penn'orth.